How to animate the particle hair size?
I'm not exactly sure if I understand your question correctly, but you can easily animate particle scale:
In your particle settings, under &quot;Render&quot; right-click on &quot;Scale&quot; and choose insert keyframe. Alternatively, you can click on the small dot right to the box.
Then, go forward in your timeline, insert a new value for &quot;Scale&quot; and again, insert a keyframe.

This may result in something like this: